1. Field of the Invention
The present invention relates to a fuel control apparatus for an internal combustion engine, and more particularly to a fuel control apparatus which is capable of performing fuel control, during a startup phase, corresponding to heavy fuel.
2. Description of the Related Art
When heavy fuel which is not excellent in volatility is used in an internal combustion engine, the internal combustion engine has such a tendency that an air-fuel ratio inclines to a lean side to make a combustion state become worse along with an increase in the fuel amount adhered to an inlet port. In particular, when catalyst warm-up control is carried out by retarding ignition timing and leaning the air-fuel ratio for the purpose of rapidly activating a catalyst during an engine cold state startup phase, and so forth, the leaning of the air-fuel ratio by the catalyst warm-up control and the leaning of the air-fuel ratio by use of the heavy fuel coincide with each other. As a result, there arises such a problem that the idling revolution becomes unstable as the combustion state deteriorates. In order to prevent such malfunction from being caused, the control corresponding to the heavy fuel is performed. In the heavy fuel corresponding control concerned, for example, the fuel amount is increased by setting an increased correction coefficient of the fuel injection amount whenever miss firing is judged based on a revolution situation of the internal combustion engine. As a result, the leaning due to the use of the heavy fuel is suppressed, thereby stabilizing the idling revolution.
However, when the cold state startup is continuously performed, from the second time startup, the fuel amount supplied to a cylinder is increased by evaporation of the adhered fuel along with abrupt temperature rise in the inlet port and the inlet valve immediately after the startup in addition to an increase in the fuel amount which is generally performed immediately after the startup. As a result, in spite of the use of the heavy fuel, no judgment for the miss firing is made, and no heavy fuel corresponding control is also performed. Though non-performing of the heavy fuel corresponding control results in that the leaning of the air-fuel ratio due to the use of the heavy fuel is caused, deterioration in the combustion state due to the leaning of the air-fuel ratio does not become apparent while an increase in the fuel amount immediately after startup is performed. Thereafter, when an increase in the fuel amount supplied immediately after startup is stopped, and the operation is completely shifted to the above-mentioned catalyst warm-up control, a combustion state deterioration resulting from the non-performing of the heavy fuel corresponding control appears to cause the unstable idling revolution.
As a measure for coping with such malfunction, it is expected, when consecutive re-startup as described above is judged, to perform an alternative control, for example, the control for performing increasing correction or the like in the fuel injection amount instead of the heavy fuel corresponding control. Various kinds of techniques for judging the re-startup of the internal combustion engine have been proposed by for instance Japanese Unexamined Patent No. 2002-70631 (hereinafter referred to as “Patent document 1”). Patent document 1 discloses a control apparatus for gradually changing fuel injection timing and for judging fuel properties based on the amount of change in fuel injection timing until the amount of change in engine speed reaches a predetermined value. In this control apparatus, when a temperature difference between a cooling water temperature and an intake air temperature at startup of the engine exceeds a re-startup judgment value, the re-startup (i.e., the above-mentioned consecutive re-startup) is judged, and the last judged value of fuel properties is applied, thereby omitting the processing for judging the present fuel properties.
Thus, it is thought that when the consecutive re-startup is judged based on the technique disclosed in Patent document 1, this case is regarded as a situation in which no heavy fuel corresponding control is performed due to impossibility of the miss firing judgment, and the fuel amount is increased by performing the above-mentioned alternative control instead of the heavy fuel corresponding control.
The technique for judging the consecutive re-startup disclosed in Patent document 1 takes notice of that the temperature difference between the cooling water temperature and the intake air temperature at the startup phase changes in correspondence to a time interval from the last engine stop to the present start-up. With this judging technique, however, the situation in which the miss firing judgment becomes impossible due to the supply of the evaporated fuel to the cylinder is merely indirectly guessed. It is impossible to make a precise judgment directly reflecting the rise of the temperature in the inlet port and the inlet valve, which was the original cause of the malfunction. Thus, no alternative control is performed in spite of the situation in which no heavy fuel corresponding control is performed due to the impossibility of the judgment for the miss firing. As a result, there is anxiety in which the instability of the idling revolution due to combustion state deterioration cannot be suppressed.